An antiresonant ring interferometer (ARR) is an optical interferometer in which a beamsplitter (BS) separates an input beam (Ein) into two beams (clockwise and counter clock-wise). After propagation inside the ring, and transmission or reflection by the beamsplitter again, the new pairs of fields interfere, giving rise to the final reflected and transmitted output fields (Er and Et).
The ARR can be treated mathematically as a 4-port optical splitter whose transfer matrix is given by
            M      ARR        =          ⌊                                    t                                r                                                              -              r                                            t                              ⌋        ,where r and t are the amplitude reflectance and transmittance, respectively. Moreover, as two of the ports are connected by the ring, the reflected and transmitted fields from the ARR are given by
      ⌊                                        E            r                                                            E            t                                ⌋    =            M      ARR      2        ⁢                  ⌊                                                            E                2                                                                                        E                1                                                    ⌋            .      
In the particular case where
            ⌊                                                  E              2                                                                          E              1                                          ⌋        =          ⌊                                    0                                                              E              in                                          ⌋        ,            then      ⁢                          ⁢              ⌊                                                            E                r                                                                                        E                t                                                    ⌋              =                  ⌊                                                            2                ⁢                t                ⁢                                                                  ⁢                r                                                                                                          t                  2                                -                                  r                  2                                                                    ⌋            ⁢              E        in              ,implying that after recombination at the beamsplitter, the reflected fields interfere in phase, whereas the transmitted fields are of opposite phase. Finally, identifying R=r2 and T=t2 (R and T being the reflectance and transmittance of the beamsplitter), the global reflectance and transmittance of the ARR result as RARR=4RT and TARR=|R−T|2.
ARRs have already been used in laser systems, such as the one disclosed by U.S. Pat. No. 3,869,210. In the said laser systems, the ARR are used, along with other elements, for different functions, such as combining two independent laser sources, optical pulse switching or laser mode locking.
An Optical Parametric Oscillator (OPO) is a light source comprising an optical resonator and a nonlinear optical crystal pumped by a laser. It converts an input laser wave (with a frequency wpump) into two output waves of lower frequency (wsignal, Widler), where wpump=Wsignal+Widler, by means of the nonlinear optical interaction known as parametric down-conversion. The gain allows the resonating wave to oscillate in the optical cavity, compensating the loss that the resonating wave experiences in each round-trip. A main feature of OPO is that wavelengths can be varied over a wide range by altering the phase-matching condition, kpump=ksignal kidler, of the nonlinear optical crystal, where kpump,signal,idler are the wave vectors of the pump, signal and idler, respectively. For instance, one can change the crystal temperature, crystal orientation or optical length of the resonator.
Many OPOs are known, such as those disclosed in US 2007/0291801A1 or US2005/0243876 A1, but in many cases they suffer from elaborate designs, non-optimum output coupling, and wavelength dependence of extracted output power.
There is thus a need for state-of-the-art optical parametric oscillators with a high stability, a wide wavelength range, and optimized output power.